spl: if MMCSD_MODE_RAW fails, try MMCSD_MODE_FS

Since commit fd61d399:
        spl: mmc: add break statements in spl_mmc_load_image()
RAW and FS boot modes are now exclusive again. So, if MMCSD_MODE_RAW fails, the
board hangs. This patch allows to try MMCSD_MODE_FS then.

It has been tested on a beaglebone black to boot on an EXT partition.

Signed-off-by: Guillaume GARDET <guillaume.gardet@free.fr>
Cc: Tom Rini <trini@konsulko.com>
Cc: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Paul Kocialkowski <contact@paulk.fr>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Matwey V. Kornilov <matwey.kornilov@gmail.com>
Acked-by: Nikita Kiryanov <nikita@compulab.co.il>